Once the House votes on impeachment, are they out?

"In the case of the trial of a sitting US president, the Chief Justice of the Supreme Court becomes the presiding officer of the Senate as it sits as a Court of Impeachment," Professor Jackman said.

"Members of the House of Representatives — who supported the motions to impeach the president — are the prosecutors, and the president can bring in essentially whoever he wants to be his defence team."

Professor Jackman said in the case of the impeachment trial of Bill Clinton, he brought in his own lawyers.

The Senate can also decide if it wants to hear direct testimony from witnesses.

"In the case of Bill Clinton they heard video testimony from various witnesses including Monica Lewinsky," he said.

What happens next?

Once the trial is done, the Senate casts a vote on whether they believe the president is guilty or not. So the House impeaches, but the Senate is the body which convicts or acquits. Conviction means removal from office.

But it's not just a simple majority vote, which could be prone to partisan results.

"It's got to be a two-third majority (that) has to vote guilty," Professor Jackman said.

Has anyone been impeached and convicted before?

Bill Clinton was impeached in 1998 and went on trial in 1999, but was acquitted and remained in office.

The process of impeachment started during Richard Nixon's term in 1974, but he resigned from office to avoid the proceedings.

How long before the president is kicked out?

Because it's never happened, we don't really know.

"If the Senate returned a guilty (verdict) … all the Constitution says is that the president is removed from office and this has never been done," he said.

"I imagine it would essentially be instantaneous. On the same day [the president was removed] the vice president would be sworn in."

So, the president is impeached. Is he cleared of all wrong doing?

"This is the interesting thing," Professor Jackman said. "The criminality is a separate matter.

"The right way to think about it is impeachment and a Senate trial process is about the institution of a presidency. It's a process ... to restore good government in the United States.

"The question about Nixon was would he then face criminal liability even though he had resigned as president, and that's why he was granted a pardon by Gerald Ford — quite controversially at the time by the way — but that … made him immune from criminal prosecution because that is still on the table and the Constitution is quite explicit about that."

Do you have to commit a crime to be impeached?

Not exactly. The rules have pretty vague language.

"It's this vague language of high crimes and misdemeanours. That's deliberately vague and so it's possible there may be criminal acts involved or there may not be," he said.

Is the system independent?

Not really.

Professor Jackman's view is that the process is "inherently political".

"There is this Constitutional language, we talk about impeachment and a trial and it's got this very legal [terminology] and it sounds like a criminal matter, but ultimately these are political bodies."

He said the respective cases of Nixon and Clinton show the survival of a president can come down to a political calculation.

"[In Nixon's case] you had a Republican president with Democratic majorities in both the House and Senate [but] his wrongdoing was so egregious he had even lost substantial numbers of Republicans. He looked at that set of cards and resigned.

"Clinton on the other hand, a Democratic president, was impeached by a Republican House of Representatives and it went to a Republican-controlled Senate as well but there was just no way there was ever going to be 67 guilty votes."

So, why is there talk Mr Trump could be impeached?

Speculation has been mounting over accusations of obstruction of justice.

The President says he knew his former national security adviser, Michael Flynn, had lied to the FBI and that was one of the reasons he sacked him back in February.

"I think this is reaching the level where something ought to be done," he said.

Stanford Law professor, David Sklansky told The World Today Mr Trump had put himself legally at risk.

"Well I'd say he's put himself even more legally at risk than he was before because the key to a prosecution for obstruction of justice or for impeachment based on obstruction of justice would be proving that the President had corrupt intent," Professor Sklansky said.

"And if he knew when he asked then-director James Comey of the FBI to quash the Flynn investigation that Flynn had lied to the FBI, and if Trump told Comey that the only reason that he had fired Flynn, was that Flynn had lied to Vice President Pence, that does tend to provide further evidence of corrupt intent."